APPLICATION NOTE

Characterizing 2D Materials
with RISE Microscopy

Ute Schmidt1, Ann-Katrin Unger2, Magdalena Eder2, and Olaf Hollricher1
1

WITec GmbH, Lise-Meitner-Str. 6, 89081 Ulm, Germany
fon +49 (0) 731 140 700, fax +49 (0) 731 140 70 200
info@WITec.de, www.WITec.de

2

WITec GmbH, Ulm, Germany
Zeiss Microscopy GmbH, Oberkochen, Germany

Introduction

Characterizing Structural and Electronic Properties of 2D Materials Using
RISE Correlative Microscopy – ZEISS Sigma 300 with WITec Confocal Raman Imaging

The mechanical isolation of single-layer
graphene with the help of adhesive tape [1]kicked off the rapid growth in research on
materials consisting of only one or a few layersof atoms, defined as 2D materials. In addition
to graphene, the most prominent example of asingle layered material, other 2D materials suchas transition metal dichalogenides (TMDs) haveattracted attention due to their exceptional
electronic and optical properties. Graphene [1,2] and TMDs [3, 4] have a layered structure incommon and experience significant changes
in their properties with layer thickness, making
them very attractive materials for electronics
design [5]. Their phononic and optoelectronic
qualities have been shown to enable ultrafast
carrier dynamics and ultra-sensitivity in the
UV-VIS, IR and THz frequency ranges [4, 5]. For
applied research and their integration withelectronic circuits and sensors, the quality of
the 2D materials is of great interest in termsof defects such as wrinkles, folding or latticemismatches [6-8]. Furthermore the phononicand electronic properties of double-layer graphene are strongly influenced by the stacking
disorder [9].
The characterization tool of choice for thesetypes of 2D materials should be non-destructive, fast, with high resolution and capable ofproviding detailed structural and electronic
information. This implies the use of more than
one characterization technique to obtain athorough understanding of their attributes for
specific applications.
This paper provides insight into how RISE
Microscopy (Raman Imaging and ScanningElectron Microscopy), a combination of optical
spectroscopic imaging (Raman/Photoluminescence) and scanning electron microscopy
techniques (SEM), contribute to an enhanced
characterization of such 2D materials.

Methods
Scanning Electron Microscopy is known for
its ability to characterize surfaces from themillimeter range down to atomic resolution,
allowing overview images of the sample andnanometer scale investigations of regions ofinterest. This technique enables the highest

Fig. 1: ZEISS Sigma 300 RISE, a combination of ZEISS Sigma 300 and a WITec confocal Raman microscope

resolution imaging of samples in order toreveal their defects. The ability to work at low
acceleration voltages or in low vacuum modeeliminates the necessity of coating the samples
with metallic films for charge prevention during SEM imaging. Thus it is possible to image
sensitive 2D materials without any specificsample preparation or modification.
Confocal Raman imaging is an establishedspectroscopic imaging method used forcharacterizing of the chemical and molecularcomponents of a sample with diffraction-limited resolution. It has proved to be of great value
in identifying the distinct spectra obtained
from single, double, triple and multi-layered 2D
materials. Furthermore, Raman imaging is by
now routinely used to determine strain, doping
type and level, stacking, chirality and disorder ingraphene and TMDs [6,7,9,10]. All of these properties can be extracted from Raman spectroscopy and imaging and can be complemented
with other techniques.
RISE Microscopy is the combination of confocal
Raman Imaging and Scanning Electron Microscopy in one instrument. It incorporates thesensitivity of the non-destructive, spectroscopic
Raman technique along with the high-resolution imaging capabilities of the electron microscope. Additionally, RISE Microscopy allows thecharacterization of these sensitive materials
inside the vacuum chamber without exposure
to air. In contrast to existing combinations inwhich single Raman spectra are typically collected from areas several micrometers in size, theRISE combination allows for diffraction-limited

confocal Raman imaging of the same sample
position from which the SEM image was taken.
It can also generate 3D-images and depth profiles to visualize the distribution of the molecular compounds within a sample volume. Bothanalytical methods are fully integrated into the
RISE Microscope. A precise translation stage
automatically transfers the sample inside themicroscope‘s vacuum chamber and re-positions
it between measurements. The integrated RISE
software carries out the required parameter
adjustment and component alignment. The
acquired results can then be correlated and theRaman and SEM images overlaid.

Experimental Conditions
All presented SEM and Raman measurements
were performed with ZEISS Sigma 300 RISE , afield emission scanning electron microscope(Fig. 1). All SEM images were acquired underhigh-vacuum conditions using low acceleration
voltages. Both, the Inlens Duo detector and theSE detector were used to image the studied 2D
materials.
The WITec confocal Raman microscope Apyron
integrated into ZEISS Sigma 300 is equippedwith a 100x vacuum objective (NA = 0.75) anda 532 nm excitation laser. Detailed imaging
parameters are provided in the corresponding
figure captions.

Results and Discussion
The power of RISE microscopy will be discussed based on three different 2D materials. In
the first section RISE imaging of CVD-growngraphene transferred on Si/SiO2 substrates are
discussed. RISE microscopy reveals structure
as well as crystalline and exciton dynamics of
few-layered transition metal dichalcogenides(TMDs). In a second section Raman SEM imagesof CVD-grown MoS2 crystals are presented.
A correlation of RISE and photoluminescence in an SEM is shown with the example of
CVD-grown WS2 crystals deposited on Si/SiO2
substrate.

RISE imaging of CVD-grown graphene
The most prominent Raman bands of monoand bi-layer graphene are the G band at 1582
rel.cm-1 and the G´ band at about 2700 rel.
cm-1. In the case of a disordered sample or at
the edge of a graphene sample, the so-called
disorder-induced D band arises at around 1350
rel.cm-1. From these three Raman bands mostof the electronic and phononic properties ofgraphene can be derived [11]. Furthermore thepositions and intensity ratio of IG/IG´ reveal thenumber of layers in a graphene sheet, whereas
the intensity ratio IG/ID shows the degree ofdisorder in graphene.
Fig. 2a shows the SEM image of a twistedbi-layer graphene flake created by chemical
vapor deposition on a 300 nm thick Si/SiO2
substrate. The color-coded marks in this image
denote positions from which single Raman
spectra were acquired (Fig. 2b). The red spectrum corresponds to the Raman spectrum ofmonolayer graphene according to the position
and intensity of the G band, the narrow shapeand the blue-shifted G´ band. The presence of
the D band in this spectrum indicates that themonolayer has a high defect density. In the blue
spectrum the intensity of the G band is muchhigher than the red-shifted intensity of the G´band corresponding to an AB stacked double
layer of graphene[9]. The green spectrum,
measured on triple-layer graphene shows anarrow G´ band, which corresponds to an ABAstacking[12]. Thus, the gray shades in the SEM
image correspond mainly to an increase in thenumber of graphene layers, indicated by theintensity of the G band in the Raman spectra.
With conventional Raman in SEM, which only

Figure 2: Surface sensitive secondary electron SEM image of twisted bi-layer graphene acquired with the Inlens
Duo detector under high vacuum conditions at 2 kV acceleration voltage and color-coded Raman spectra
acquired from the marked positions in the SEM image. Each spectrum was acquired with an integration timeof 500 ms at 2 mW laser power.

Figure 3: RISE image of a grain of twisted bi-layer graphene obtained by overlaying the SEM image with the color-coded Raman image. Raman imaging parameters: scan size 45x45 µm2, 150x150 Raman spectra, integration
time 50 ms/spectrum at 5 mW laser power.

allows for the acquisition of single Raman spectra, further analysis of graphene is not possible.
Raman imaging of the same sample area reveals far more details about the electronic andphononic properties of the double-layer graphene. The RISE image (Fig. 3) shows that theentire substrate surface is covered by grapheneand that there are zones of different layers as
revealed by the measured G and G´ Raman
bands for graphene. By analyzing the intensityand full width at half maximum (FWHM) of
these two characteristic Raman bands, it ispossible to determine the stacking order andtwist angle of the analyzed twisted double-layer graphene grain [7-9].
Fig. 4 shows the variations in peak intensities
of IG/IG´ and changes of the FWHM of theG´ Raman band together with the associatedvariations of the twist angles and stacking
orders. To highlight these peak intensity
variations and shifts, the Raman spectrum of
mono-layer graphene is added as a dashed

Figure 4: RISE images and Raman
spectra highlighting areas withdifferent stacking orders andtwist angles. As reference theRaman spectrum of monolayer
graphene is shown as dashedline in each spectrum.

line. The correlations of Raman bands andtwist angles are summarized in table 1. Theinterference between two rotated graphene
layers generates a superlattice with an angle
dependent wavevector that gives rise to van
Hove singularities in the electronic density ofstates and activates phonons in the interiorof the graphene Brillouin zone. By controlling
the twist angle between two graphene layers,
the electronic and optical properties of twisted
double-layer graphene can be modified to
specific needs.

IG
I

,
G

FWHM

I (ML)

2ML

2Ml

2ML

2ML

>> ML

>> ML

I (ML)

< ML

>> ML

~ ML

< ML

~ ML

>> ML

> ML

2ML

>> ML

Raman bands[rel. cm-1]---

R` @ 1625 ---

---

---

R @ 1514///

Twist Angle Θ[degree]

---

3-8

~ AB

0-3

12

Multilayer

,
G

ML

> 20

Table 1: Correlation of graphene Raman band intensities with the twist angle between double-layers graphene.
ML = intensity of G or G´ Raman band corresponding to a monolayer graphene.
R`= Raman band which arises only at small rotation angle between the double layer of graphene.
R = resonant Raman band which arises at a rotation angle of 12 degrees when the double- layers graphene are
excited at 532 nm.

A practical implementation of graphene
requires the development of technologies for
growing large graphene samples of high quality. A review of the various CVD growth methods
is provided in reference [13]. The size and qualityof such monolayer graphene can be studied
using RISE microscopy.
Fig. 5 shows the SEM image of a large scale produced monolayer of graphene CVD-deposited
onto Cu and transferred to a 300 nm thick Si/
SiO2 substrate using a PMMA interface. During
the transfer process, defects are induced in thegraphene monolayer which can be visualizedwith the SEM. Beside the dark cracks, small dark
regularly shaped areas are visible, which correspond to double layers of graphene. In the brightareas of this SEM image areas with wrinkles arevisible. A color-coded Raman image acquired
from an area of 20x20 µm2 is shown in Fig. 6a
together with the corresponding spectra in Fig.
6b. The RISE image is presented in Fig. 7.
The intensity ratio of IG/IG´ clearly identifies
the dark regularly shaped areas as double
layers of graphene (pink spectrum in Fig. 6a).
Green colored areas in the Raman image from
Fig. 6b denote positions with a high densityof wrinkles in the SEM image and a lower
intensity ratio of IG/ID peaks. The red and blue
colored areas and spectra describe a monolayerof graphene. The small difference in spectra isassociated only with a slight red shift of theblue spectrum, which may be associated withdifferent stress-zones in the graphene layer.

Fig. 5: Surface sensitive secondary electron image ofa large monolayer graphene sample acquired with
the Inlens Duo detector at 1 kV acceleration voltage.

ab

Figure 6: Color-coded Raman image of a large monolayer graphene sample (b) and the corresponding
Raman spectra (a) evaluated from a spectral arrayof 100x100 Raman spectra acquired from an areaof 20x20 µm2, with an integration time of 200 ms/
spectrum at 2 mW laser power.

Figure 7: RISE image of the large monolayer graphene sample obtained by overlaying theRaman image from Fig. 6a onto the SEM image from Fig. 5.

RISE imaging of MoS2
CVD-grown monolayers of MoS2 form triangular two-dimensional crystals. Twin crystals of
MoS2 on SiO2/Si appear in the SEM image as
star-shaped forms (Fig. 8a).
The Raman spectra of a MoS2 monolayer show
the characteristic E‘/E2g and A‘1/A1g Raman
band modes of MoS2 at 386 and 403 rel.cm-1,
respectively (Fig. 8b). With an increasing number of layers the two Raman bands drift apartdue to in-plane and out-of-plane (inter-layer)
vibrations. Here the red spectrum indicates themonolayer of MoS2, while the blue and aqua
spectra signify double and multi-layer MoS2.
The green spectrum is associated with edges
and defects, respectively.

ac

The spectral information was used to createa Raman image which was overlaid onto the
SEM image to result in a RISE image (Fig. 8c).
The grain boundaries visible in the SEM image
correlate perfectly with the areas where Raman
signals indicate crumpled or overlapping edges,
highlighting the effectiveness of RISE imaging
for characterizing MoS2 crystals (Fig. 8d).

Figure 8: Secondary electron image of MoS2 crystals
imaged with the Chamber SE detector at 2 kVacceleration voltage under high vacuum conditions(a), Raman spectra evaluated from a spectral arrayof 100x80 Raman spectra acquired from an area of
22x18 µm2, with an integration time of 50 ms/spectrum at 5 mW laser power (b), RISE image of MoS2
crystals (c) and high resolution RISE image of thegrain boundaries between single MoS2 crystals (d).

db

RISE imaging of WS2
Here we show the analysis and RISE imaging of
WS2 crystals. The high-resolution SEM images
were acquired under low vacuum conditions
at 5 kV acceleration voltage using the Inlens
Duo detector and the secondary electron (SE)detector, respectively (Fig. 9 a, b). Both SEM
images show the typical triangular shape of
the WS2 crystals. In the SEM image acquiredwith the Inlens Duo detector, the WS2 crystals
appear as bright triangles, whereas the SE
detector shows these crystals as dark material.
Furthermore, the Inlens Duo detector is moresensitive to the thickness of the material,
revealing defects or additional layers within
the crystals.
The Raman spectrum acquired with the 532 nm
excitation laser (Fig. 10a) shows a strong WS2
Raman signal at 350 rel.cm-1 [14]. Researchers
were able to divide this peak into three sub-peaks [15]. They associated a signal at 344 rel.cm-1
with the in-plane E‘/E2g Raman mode. The outof-plane A‘1/A1g Raman band appears at thesame relative wavenumber of 417 rel. cm-1 asin the monolayer of MoS2. Using this spectral
band a Raman intensity map was generated
that highlights a quantified increase of theRaman band intensity in the region of a double
layer (green area Fig. 10b). All the other crystals
appear homogenous as monolayers of WS2. By
combining the SEM and the Raman images,
the RISE image from Fig. 10c can be generated.

Figure 10: Raman spectrum of WS2 (a) and Raman
intensity map of the Raman band at 717 rel. cm-1 (b).
Raman image acquisition parameters:
100x100 Raman spectra acquired from an area of
15x15 µm2, with an integration time of 60 ms/spectrum at 2 mW laser power. (c) RISE image of the WS2
crystals.

ab

Figure 9: SEM images of WS2 crystals acquired with the Inlens Duo (a) and the Chamber SE (b) detectors at 5
kV acceleration voltage.

a

c

b

For this group of 2D materials the information obtained through the combination oftechniques is even more valuable as the transition of indirect to direct semiconductor whengoing to a single layer gives rise to pronounced
photoluminescence (PL) [4]. Fig. 11a shows thetwo extreme PL spectra acquired from the analyzed sample area together with the PL image(Fig. 11b).
The WS2 crystals display strong PL in the range
630-650 nm, with variations in position withthe size of the analyzed crystals. An overlay of
the SEM images with the PL image is shownin Fig. 12. This strong PL is observed only in the
single monolayer crystals due to the direct
band gap in the monolayer of WS2.

a

a

b

Figure 11: PL spectra acquired from WS2 crystals (a) and the distribution of the PL shift over the analyzed sample
area (b). The PL spectra were acquired from an area of 15x15 µm2, with an integration time of 35 ms/spectrum at
2 mW laser power.

Figure 12: Overlay of the two SEM images from Fig. 9 with the PL image.

b

Summary
ZEISS Sigma 300 RISE is a versatile tool for high resolution SEM imaging of sensitive 2D materials due to its ability to acquire images at low acceleration voltages
and low beam currents. Furthermore, charging and beam damage can be easily
prevented by working under low vacuum conditions without losing high resolution capabilities. By attaching the confocal Raman microscope to a standard portof the vacuum chamber an easy workflow between SEM and Raman imaging isassured. The added value of Raman and PL imaging for the characterization of
2D materials is presented in this study.

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